| The use of probiotics in aquiculture draw people attention more and more in nowadays. It is universly accepted that probiotics as an alternative mean of antibiotics. Now most of commercial probiotics used for terrestrial animals such as Lactobacillus spp. and Bacillus spp. are being used in aquaculture, nevertheless, the safety and efficiency of these exogenous probiotics are still questionable in aquaculture. The intestinal microorganisms of aquatic animals have a crucial impact on the host, and therefore, the intestinal microorganisms may have the potential to be used as probiotics. Grass carp(Ctenopharyngodon idellus) is one of the most important freshwater species in China. This research based on the preliminary analysis on the microflora of the intestinal tract of grass carp, and isolated the autochthonous gut bacteria from intestinal tract of grass carp and studied the effects of single or combined use of dietary autochthonous gut bacteria on immune responses and resistance to disease of grass carp. The achievements of this research are as fellows:1. The study investigated the composition of the intestinal autochthonous bacteria between feeding group and no-feeding group. A total of 295 strains were isolated from the no-feeding group, and three hundred and sixteen strains were isolated from the feeding group. The results showed that the autochthonous gut bacteria of grass carp from two groups were similarity composed of the genus of Aeromonas, Shewanella, Vibrioand Pseudomonas. The dominant species in the intestinal tract of two groups was similar, Aeromonas and Shewanella was the dominant species and Vibrio, Pseudomonas were the secondary dominant species, which indicated that grass carp have specific selective of microflora in the intestinal tract.2. Three strains of intestinal dominant bacteria A-1, A-2 and A-7 were selected after hemolytic assays which were used to exclude potential pathogens. The safety of the selected isolates was evaluated and showed that no probable pathogenic in hemolytic assay. Then, they were identified as Shewanella xiamenensis A-1, Shewanella xiamenensis A-2 and Aeromonas veronii A-7 by 16 S r DNA gene and biochemical tests.3. Experiment was conducted to investgate the effects of feeding single probiotic(A-1, A-2 and A-7) and mixed probiotics to grass carp on immune function and disease resistance. The grass carp were fed single probiotic(A-1, A-2 and A-7) and mixed probiotics in diet at a concentration of ~108 cfu/g for 28 days. At the end of the 28-day feeding experiment, the survival of grass carp against Aeromonas hydrophila was challenged for 14 days. Results showed that responses of several non-specific immune parameters of groups fed probiotics were enhanced; i.e. all probiotic groups led to significant enhancement of both respiratory burst, phagocytic and lysozyme activities than that of control activity. The complement C3, total serum proteins, albumin and globulin levels were also significantly improved on day 14 of feeding regime with probiotics compared to the control. Moreover, the expression of four immune-related genes(IL-8, IL-1β, lysozyme-C, and TNF-α genes) was significantly up-regulated in grass carp fed probiotic diets compared to control group on days 7, 14, and 28. The cumulative mortality of grass carp, experimentally challenged with A. hydrophila, was reduced in groups fed probiotics, i.e. A-1=46.67%, A-2=33.33%, B=53.33% and mixed=26.67%, respectively, than that of controls(80%) after 14 days of post-infection. |
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